Sperm, or male germ cells, are expelled during ejaculation in the form of a fluid called semen. Ejaculation inside the female is essential for conception to occur. But how long do sperm have to travel inside the body and reach the egg, the female germ cell?
Sperm survival is defined by a series of parameters, but two stand out: temperature , which must be between 37 ° C and 37.5 ° C; and the degree of acidity of the environment (pH 7-7.5). In this sense, the vagina is a hostile environment for male reproductive cells, especially outside the ovulatory period. During this time, the sperm literally burns to death in a few minutes, since the vaginal pH is below 6. On the days of ovulation, it rises 1 or 1.5 points, which allows them to last from 2 to 16 hours . If they are ejaculated outside, at a temperature of 15ºC, they remain vital for 1 or 2 days, and much less if it is hotter.
In most higher vertebrates, mature sperm have two distinguishable parts, a head and a tail. It is in the head where they carry the genetic information, and a tail or flagellum, which they undulate to propel themselves inside the body of the female. Sperm have only 23 chromosomes, or half the usual number. When a sperm cell unites with the egg, which also has 23 chromosomes, the resulting 46 chromosomes determine the characteristics of the offspring. Sperm also carry the X or Y chromosome that determines the sex of the future child.
As for the flagellum, the activation of the movements of the tail is part of the training process, in which the sperm undergo a series of cellular changes that allow their participation in fertilization . A fundamental change that occurs during capacitation is the alkalinization of the sperm cytoplasm, in which intracellular pH levels increase, particularly in the flagellum. This process, which is driven by the rapid movement of protons out of the cell through ion channels in the flagellum, underlies the activation of the tail.
Proton channels in sperm flagella are primed to open by the presence in the female reproductive tract of a substance known as anandamide, which is believed to occur in high concentrations near the egg. Upon reaching an egg, the enzymes contained within the sperm acrosome are activated, allowing the sperm to pass through the thick layer that surrounds the egg (the zona pellucida); This process is known as the acrosome reaction . The sperm membrane then fuses with that of the egg, and the sperm nucleus is transported to the egg.
Sperm deposited in the female’s reproductive tract that do not reach the egg die. But sperm can also be stored in a frozen state for months or even years, retaining its ability to fertilize eggs when thawed.
